Marilú Torres' knees hurt when they swell, her varicose veins are a constant bother, and cataracts are slowly stealing her vision away. Even so, the 72-year-old hits the streets looking for work every day, although she has to walk much further to find clients than she used to.

"This year is my golden anniversary as a sex worker," laughed Marilú, who became a prostitute as a young widow with no other means of feeding her three children. "I'm not complaining, I've learned a lot about life in this job. But the old bones are turning to dust like the mummies, and it would be nice to be able to stop soon."

She was talking in an abandoned sports museum in the run-down colonial city centre, which is being adapted as a shelter for elderly prostitutes who work the area.

There are about 120 of them working the district, charging a standard 70 pesos (£3.30) a job, unless they are desperate enough to accept less, or are offered a little more by a client who insists on not using a condom.

Many are homeless, turning tricks for the price of a meal or, if they are lucky, the cost of a night in a seedy hotel as well. Many are also spurned by the children they went into prostitution to provide for.

The new shelter aims to give these survivors of the sex trade a chance to retire in a country where the social security system is woefully inadequate for most, and virtually non-existent for women such as these.

It should provide a guaranteed bed for up to 70 of them, as well as food, healthcare, and workshops where they can learn how to make things they can sell instead of their bodies.

"It is a beautiful thing to have a place to drop," said Ms Torres, prompting nods from workmates also taking a break in the would-be shelter. "We are going to make this a home."

The rather grand, if dilapidated, building arranged around a large patio has been loaned free of charge by the city council. Renovation and initial maintenance (which will cost about £150,000) is in the hands of private NGOs and a group of prominent feminists who have taken on the project as a personal cause. They hope it will be ready in six months.

"It will be wonderful not to have to walk the streets, not to have to feel the way people look at you," said Elena Román, 65. "I hate those stares. I feel so ashamed."

With only a decade on the job, triggered by her decision to leave a violent husband, Ms Román is a relative newcomer to the business.

Others, such as 70-year-old Florencia Sánchez, have a lifetime of experience. She began as a 13-year-old runaway, took a two-decade break while married, and has somehow managed to keep her profession secret from her eight children.

Today Ms Sánchez is among the hardest-working in her age bracket, desperate to earn the cash she needs to bring up her two teenage grandchildren, who were left in her charge when their mother died in an accident.

"I have no choice," she said without drama or self-pity. "They are my children now and I want them to study."

The women joked easily about how to deal with clients with bad breath or smelly feet, told hair-raising stories of encounters with violent customers, and bemoaned the fact that business wanes with each new wrinkle.

They now rely on younger men looking for maternal tenderness, grandfathers seeking familiarity or psychological therapy, and those of any age who feel mature prostitutes have more experience and are less risky than their younger competitors.

"I think the most important thing for clients is that you smell clean," said Ms Román, looking the epitome of housewifely respectability in her buttoned-up blouse and jumper.

Ms Torres, who delights in her prowess at double entendre and her impressively fresh flirting style, said she no longer bothers to dress in sexy clothes. "At my age?" she chuckled.